The present invention relates to a pressure foot used in a printed circuit board drilling apparatus for drilling a printed circuit board with a drill.
A printed circuit board is processed through several steps, i.e. usually drilling, through-hole copper plating, patterning, etching and stripping, printing for solder resist, gold plating on contact fingers, solder coating press for blanking and finishing, electrical inspection, final inspection, flux coating, shipment, and mounting. The drilling step is the first step for processing a printed circuit board.
Therefore, the drilling process requires a high precision and a high quality.
In a drilling process of a printed circuit board, for preventing any protrusion from being formed at hole ends of the printed circuit board, an upper or entry board and a lower or back-up board are stacked on the upper and the lower surfaces of the printed circuit board, respectively, and a pressure foot is arranged for pressing these boards at regions near the hole.
For example, when a lower board of 0.063 inch thick, three printed circuit boards of each 0.063 inch thick and an upper board of 0.04 inch thick are stacked one upon another and clamped together, and then drilled in a direction from the upper board toward the lower board until the lower board is drilled by depth of 0.04 inch, the depth of the drilled hole in the stacked boards becomes 0.269 inch.
When the diameter of the hole to be drilled is, for example, 0.047 inch, the aspect ratio (ratio of hole depth Lh to hole diameter D) becomes 5.7.
In such a case where the diameter of the hole to be drilled is rather large, and the aspect ratio is rather small (below 6-8), the printed circuit boards can be drilled by moving a drill from a drilling starting point A to a drilling end point B with a single drill feed rate (FIG. 1).
On the other hand, when the diameter of a hole is, for example, 0.031 inch, the aspect ratio reaches 8.7.
In such a case where the diameter of the hole to be drilled is small and the aspect ratio is greater, it becomes difficult to carry out the drilling from the drilling starting point A to the drilling end point B in a single motion in the manner as described above.
Namely, in the case of a greater aspect ratio, chips generated in a drilling process become difficult to be taken out and the drill flute is clogged with these chips, thereby causing the following problems:
1. Smoothness of the inner wall of the drilled hole is deteriorated; PA1 2. Drill bit wear increases, because a large quantity of heat is generated by the friction between the drill and the hole wall and the temperature of the drill is substantially raised; PA1 3. Base material (such as epoxy resins) of the printed circuit board is molten due to the raised temperature of the drill, and the molten material is adhered to the copper layer exposed to the inner surface of the hole, and when cooled, there appear smears; and PA1 4. Thrust load of the drill increases, making the drill easy to break.
At the present time, electronic packaging density of the printed circuit board has been increasing. Therefore, conductor line width on the printed circuit board is required to be finer than 0.04 inch, and consequently, the hole diameter of the printed circuit board is required to be smaller than, for example, 0.016 inch. Thus, the aspect ratio of the hole has become large and it becomes difficult to drill in a single stroke drilling.
An arrangement of the drilling apperatus for a printed circuit board described above is shown in FIG. 2 as an example.
In FIG. 2, there are shown a bed 1 of a drilling apparatus for a printed circuit board, and a table 2 which is mounted on the bed 1 movable along a direction indicated by an arrow X and moved on the table 2 by a driving means not shown. Further, drills 3 are removably held by holders 4 secured to the table 2. A column 5 is secured to the bed 1 and has a shape striding over the table 2. A carriage 6 is supported by the column 5 movably in an arrowed direction Y and moved along the column 5 by a motor 7. A saddle 8 is mounted on the carriage 6 movably in an arrowed direction Z and moved along the carriage 6 by a motor 9. Spindles 10 are rotatably supported by the saddles 8 and rotated by a motor 11. Each of the spindles has a chuck at an end portion thereof which grasps and takes out one of the drills for drilling operation.
Each of printed circuit boards W is provided with an upper board and a lower board piled on and integrally secured to each side of the base circuit board, respectively, and together fixed to the table 2 by means of reference pins P.
In the arrangement mentioned above, by moving the table 2 in the direction X and the carriage 6 in the direction Y, the position of the drill 3 held by the spindle 10 is determined relative to the printed circuit board W, and then the spindle 10 is moved in the direction Z for drilling the printed circuit board.
The spindle 10 of the printed board drilling apparatus mentioned above is structurally shown in FIG. 3.
In FIG. 3, a cylinder 12 is secured to the saddle 8, and is formed with a cavity 13 of a greater diameter and a flange 14 inwardly projecting at an end thereof. Compressed air is supplied to the cavity 13 through a pipe 15. A bearing 16 rotatably supports the spindle 10 in the central portion of the bearing, and is supported by the cylinder 12 in a fit condition. A chuck 17 is supported by the spindle at an end thereof in a manner such that the chuck can be opened and closed. A piston 18 is formed with a flange 20, which movably fits the a space 19 defined by the cavity of the cylinder 12 and the bearing 16. A pressure foot 21 is secured to an end of the piston 18. The pressure foot 21 is provided at the side surface thereof with an exhaust port 22 which is to be connected with a vacuum room of a dust collector for exhausting chips produced in the drilling process, and at the bottom surface thereof with grooves 23 for sucking air in the drilling process.
In the arrangement above, the piston 18 is moved to the lower end of the cylinder 12 by supplying air of a predetermined pressure through the pipe 15 to the space 19, and the room in the pressure foot 21 is evacuated, through the vacuum port 22.
FIG. 4 shows an air flow pattern in the pressure foot according to the arrangement above. By virtue of such an air flow, the drill 3 can be cooled and the chips adhered to the drill 3 can be removed.
It is disclosed, for example, in U.S. Pat. No. 4,340,326 to use a pressure foot for pressing printed circuit boards and drilling the same.
When a pressure foot is used for pressing, it is required to clamp the printed circuit boards at a region near the hole to be drilled for preventing the upper board and the printed circuit boards from rising up.
For this purpose, the end portion of the pressure foot has an inner diameter of about 0.4 inch, and formed with radial grooves on the end surface thereof.
Accordingly, the size of the grooves is so limited as not to satisfy a requirement for the size. On the other hand, the pressure foot is connected with a chip collector, and the air in the pressure foot is evacuated by means of vacuum throughout a drilling process. Because the chip collector has a rather large vacuum capacity and the grooves on the end surface of the pressure foot have a rather small size, the pressure in the pressure foot is always relative to the ambient atmosphere throughout the drilling process.